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United States Patent |
5,145,652
|
Veser
,   et al.
|
September 8, 1992
|
Apparatus for the removal of nitrogen burner exhaust
Abstract
Method for the removal of nitrogen from the exhausts of burners after they
are cleaned of dust or sulfur, in which the nitrogen-bearing exhaust gases
are reheated by the transfer of heat from the denitrogenated exhaust gases
in a regenerative heat exchanger.
Before they are finally heated to the temperature level of the reduction
reaction, the nitrogen-bearing exhaust gases, and, after their reduction,
the denitrogenated exhaust gases, are passed countercurrently to one
another in a cyclical alternation, through heat exchanging storage masses
in an additional catalytic converter serving as the main converter, whose
surfaces are provided with catalytically active compounds.
Inventors:
|
Veser; Kurt (Heidelberg, DE);
Muller-Odenwald; Hermann-Eugen (Mannheim, DE)
|
Assignee:
|
Kraftanlagen Aktiengesellschaft (Heidelberg, DE)
|
Appl. No.:
|
725385 |
Filed:
|
June 28, 1991 |
Foreign Application Priority Data
Current U.S. Class: |
422/171; 95/128; 165/8; 422/170; 422/172; 422/173; 422/175; 423/237; 423/239.1; 423/244.01 |
Intern'l Class: |
B01D 050/00 |
Field of Search: |
422/170-173,175
165/8
48/203,76
423/237,239,242,243
55/77-80,99,84
|
References Cited
U.S. Patent Documents
3126945 | Mar., 1964 | Kuhner | 55/6.
|
4003711 | Jan., 1977 | Hishinuma et al. | 423/239.
|
4118199 | Oct., 1978 | Volker et al. | 422/171.
|
4196170 | Apr., 1980 | Cemenska | 422/172.
|
4678643 | Jul., 1987 | Fetzer | 422/175.
|
Primary Examiner: Warden; Robert J.
Assistant Examiner: Santiago; Amalia
Parent Case Text
This application is a continuation of application Ser. No. 07/399,499,
filed Aug. 21, 1989 as PCT/EP89/0048, Jan. 18, 1989, now abandoned.
Claims
We claim:
1. An apparatus for removing nitrogen from exhaust gases of a furnace
installation comprising: means for removing particles selected from the
group consisting of dust and sulfur from exhaust gases generated by a
furnace installation; a gas heat exchanger connected to said removing
means downstream thereof, said heat exchanger being a rotating
regenerative heat exchanger having first heat exchange storage masses on a
relatively cold side adjacent said removing means and having second heat
exchanging storage masses with catalytically active compounds which are
capable of removing nitrogen oxides and adsorbing reducing agent on a
relatively hot side following the first storage masses, a second heater
downstream and in fluid communication with of said second storage masses,
means for introducing reducing agent downstream of said heater, a second
catalytic converter downstream of said reducing agent introducing means
and provided with denitration static catalytically active surfaces,
conduit means upstream of said static catalytic converter for providing
fluid communication between the static catalytic converter and said second
heater, said conduit means being in fluid communication with said means
for introducing reducing agent, means downstream of said second catalytic
converter for connecting said second catalytic converter with said second
storage masses for removing residual nitrogen oxides and adsorbing
residual reducing agent in said exhaust gases, and means for removing
cleaned gas from said first storage masses.
2. An apparatus for removing nitrogen from exhaust gases of a furnace
installation comprising: means for removing particles selected from the
group consisting of dust and sulfur from exhaust gases generated by a
furnace installation; a gas heat exchanger connected to said removing
means downstream thereof, said heat exchanger being a rotating
regenerative heat exchanger having first heat exchange storage masses on a
relatively cold side adjacent said removing means and having second heat
exchanging storage masses with catalytically active compounds which are
capable of removing nitrogen oxides and adsorbing reducing agent on a
relatively hot side following the first storage masses, a second heater
downstream of said second storage masses, means for introducing reducing
agent downstream of said second heater, a second catalytic converter
downstream of said reducing agent introducing means and provided with
denitration, moving catalytically active surfaces, conduit means upstream
of said second catalytic converter for providing fluid communication
between the second catalytic converter and said second heater, said
conduit means being in fluid communication with said means for introducing
reducing agent, means downstream of said second catalytic converter for
connecting said second catalytic converter with said second storage masses
for removing residual nitrogen oxides and adsorbing residual reducing
agent in said exhaust gases, and means for removing cleaned gas from said
first storage masses.
Description
The invention is based on a method for removing nitrogen from burner
exhausts from which the dusts or sulfur have been removed, and reheating
them, and it relates to the apparatus intended for the practice of the
method.
Methods are known for removing nitrogen from burner exhausts after the
dusts or sulfur have been removed, and reheating them by transferring heat
from the denitrogenated exhausts in a circulating regenerative heat
exchanger--a so-called gas heat exchanger--in which, after reheating to
the temperature level of the reduction reaction has been completed, a
reducing agent is introduced into the nitrogen-bearing gas stream, and
then the latter is carried over catalytically active surfaces of a
catalyst.
In these methods the exhaust gases are first preheated and then heated with
external heat to the optimum temperature level for the reduction before
they are fed to a catalyst. Statically disposed carriers bearing
catalytically active surfaces, and moving relative to the gas connections,
especially those of the exhaust from the combustion air, have likewise
become known. High-dust catalytic converters preceding the cooling of the
flue gases in the combustion air preheater, as well as those preceding the
removal of dust and sulfur, make use of the high temperature level for
performing the reduction reaction, so they have no need for reheating. On
the other hand, in the case of catalytic converters connected to the
output from the dust remover, and especially from the desulfuration--the
so-called cold-end converters--flue gas components that lead to
deactivation, such as arsenic in the case of melting burners, are removed
simultaneously with the removal of dust or sulfur, so that they will no
longer interfere with and poison the catalysts. The gases from which the
dust or sulfur has been removed are then passed through a gas heat
exchanger--with an intervening heating to dry them, if desired--for the
purpose of recovering heat from the denitrogenated gases before they enter
the exhaust stack, and transferring it back to the gases entering the
denitrogenating process, before the latter are introduced into a steam
heater or into a combustion chamber for overcoming heat exchanger
concentration, in order to heat them to the optimum reaction temperature
for the denitrogenation before the gases enter the catalyst.
Common to all these apparatus is the problem that the input of the reducing
agent must be very carefully distributed in accordance with the locally
varying flow velocities and the likewise locally varying nitrogen oxide
content in the flue gas stream. For this purpose, very complex flow
smootheners as well as a great amount of measuring instruments and
controls are needed. The requirements for a precisely metered input of the
reducing agent become more stringent as the required reduction rates
increase, since as the rate of reduction increases so does the danger that
unconsumed reducing agent may be carried over into parts of the apparatus
that follow, also increasing the possibility of undesirable reactions with
other flue gas components. After a long period of exposure to catalyst
poisons catalysts become deactivated and have to be replaced after, for
example, 5 years. The life of the catalyst is thus limited by the reducing
agent carryover, which puts stress on downstream parts of the apparatus
and causes emissions to increase.
The invention is based on the problem of performing the reduction of
nitrogen oxides in flue gases with a high degree of efficiency and without
bulky equipment, with simple methods for the measurement and control of
the addition and distribution of reducing agent in accordance with the
localized and changing concentration and distribution of the flow of the
nitrogen oxides upstream and downstream from the main catalytic converter.
For the removal of nitrogen from burner exhausts following the removal of
dust or sulphur, with reheating of the nitrogen-bearing exhaust gases by
thermal transfer from the denitrogenated gases in a regenerative heat
exchanger is characterized by the fact that, in a cyclical alternation,
the denitrogenated exhaust gases after the reduction of the nitrogen
oxides they contain, and the nitrogen-bearing exhaust gases before their
final heating to the temperature level of the reduction reaction, are
carried countercurrently to one another, in an additional catalytic
converter serving as the main converter, over heat exchanging storage
masses whose surfaces are provided with catalytically active compounds.
By this contact between heat-exchanging storage masses whose surfaces are
provided with catalytically active compounds and the nitrogen bearing and
denitrogenated gases flowing countercurrently to one another, any amounts
of reducing agent still contained in the denitrogenated gases are bound
chiefly at the entrance end to the heat exchanging storage masses whose
surfaces are provided with catalytically active compounds and at the same
time they react with residual amounts of nitrogen oxides in the exhaust
gas stream. Reducing agents bound to the heat-exchanging storage masses
whose surfaces are provided with catalytically active compounds are fed to
the nitrogen-bearing gas side. There these small amounts of reducing agent
are in contact with the full amount of nitrogen oxides, so that this
portion of the reducing agent becomes completely reacted.
Fundamentally, the method of the invention must be practiced with an
apparatus constructed in the manner of a switching heat exchanger which
includes heat-exchanging storage masses whose surfaces are provided with
catalytically active compounds. More advantageous, however, is the use of
a rotating regenerative heat exchanger, whether it has a rotating storage
mass carrier and stationary gas connections, or whether it has a
stationary storage mass carrier and rotating gas connections. In this
case, residual reduction takes place in the largely denitrogenated gases
in their direction of flow, and adsorption of any remainder of unconsumed
reducing agents takes place on the heat-exchanging storage masses whose
surfaces are provided with catalytically active compounds, while the
temperature of these surfaces diminishes, and on the nitrogen-bearing gas
side and in their direction of flow amounts of reducing agents adsorbed on
these heat exchanging storage masses whose surfaces are provided with
catalytically active compounds enter into reaction with the high nitrogen
oxide concentration of the nitrogen-bearing gases as the temperature of
these surfaces increases. In this connection it is advantageous if the
nitrogen-bearing gases are heated prior to their passage through the
storage masses whose surfaces are provided with catalytically active
compounds and the denitrogenated gases are cooled after their passage
through the said storage masses.
As an additional advantage towards a very uniformly distributed
concentration of reducing agents in the nitrogen-bearing gas stream, the
reducing agent alone or in mixture with a carrier gas is introduced into
the nitrogen-bearing gas stream upstream or downstream from the final
heating. Placing the introduction of the reducing agent upstream is
involved especially in connection with the final heating in a steam
heater, whereas in the case of final heating in a combustion chamber the
introduction of the reducing agent will be performed downstream.
The adsorption of the reducing agent residues onto the surfaces of
heat-exchanging storage masses provided with catalytically active
compounds allows the denitrogenated exhaust gases emerging from the main
catalytic converter to contain unreacted reducing agent. Furthermore, a
precise setting of the necessary amount of reducing agent can be performed
advantageously through the introduction of an additional portion of it
downstream from the main catalyst, since subsequently, in addition to the
simultaneously occurring post-reaction, any unconsumed reducing agent will
be bound by adsorption onto the heat-exchanging storage masses whose
surfaces are provided with catalytically active compounds and is
transferred to the nitrogen-bearing gas side. This addition of reducing
agent is performed only to the extent that, at the end of the
heat-exchanging storage masses whose surfaces are provided with
catalytically active compounds, the introduced reducing agent is fully
reacted or adsorbed onto these surfaces.
The process can be performed to special advantage by a gas heat exchanger
configured as a rotating regenerative heat exchanger with a layer of
heat-exchanging storage masses whose surfaces are provided with
catalytically active compounds disposed at the hot gas end, followed at
the nitrogen-bearing gas end by a heater and main catalyst in tandem, and
whose hot-gas end is connected to the main catalyst at the denitrogenated
gas end. The main catalytic converter can in this case be constructed as a
static converter or in the manner of a Ljungstrom preheater with a carrier
moving relative to the gas connections and carrying catalytically active
compounds.
By the method of the invention, wisps of reducing agent emerging from the
main catalytic converter are homogenized and then deposited by adsorption
uniformly onto the heat-exchanging storage masses whose surfaces are
provided with catalytically active compounds, and are available in this
form of deposit at the nitrogen-bearing gas end. The deactivation of the
main catalyst, which ultimately leads to an increasing leak-through of
reducing agent, can be retarded by this adsorption of the residual
reducing agent and the cyclical alternation of exposure to the
denitrogenated and the nitrogen-bearing gases and their countercurrent
flow, since the heat-exchanging storage masses whose surfaces are provided
with catalytically active compounds in this case need to satisfy only low
reduction rates. The method permits the main catalyst to be designed with
a reduced catalytically active area, i.e., in a smaller size, and to
further reduce emissions. At the same time, complex measures, especially
as regards flow smoothing and the measurement and control methods for
precise control of the input of the reducing agent through continual
measurement, can be avoided.
By the method of the invention the denitrogenation of exhausts is to be
performed while making use of residual amounts of unused reducing agents
carried in the exhaust gases issuing from the main catalyst such that the
downstream parts of the apparatus will not be impaired by byproducts of
the selective, catalytic reduction, and even at the end of a long run of
the main catalytic converter, no intolerably great leak-through of
reducing agents will escape the apparatus. Adsorptive coatings on the
heat-exchanging storage mass of the gas heat exchanger can enhance this
effect of capturing the leak-through of reducing agent and/or nitrogen
oxides as a result of the cyclical alternation of adsorption and
desorption between the denitrogenated and the nitrogen-bearing gas streams
.
An apparatus for the practice of the method of the invention is represented
diagrammatically to explain the idea of the invention.
According to the diagram, the exhaust gases to be cleaned of nitrogen are
carried from a desulfurizer 1 first through an exhaust duct 20 for
preliminary drying by a dryer 3. From the dryer 3 the exhaust gases are
then introduced through a duct 22 into a gas heat exchanger 5. The
revolving support in this gas heat exchanger has heat-exchanging storage
masses 5a on the so-called cold side, in the form, for example, of common
stacks of heating plates. On the hot side situated opposite the entrance
of the gases to be cleaned of nitrogen there is disposed in this support a
layer of heat-exchanging storage masses whose surfaces are provided with
catalyzing compounds at 5b. After leaving this layer at the hot end of the
gas heat exchanger the gases are fed through a duct 24 to a heater 7 in
order to raise the temperature of the gases to an optimum level for the
reduction reaction. The heater is warmed by external heat.
Downstream from the heater, reducing agent is fed through a line 28 to the
gases moving through the duct 26 before the nitrogen oxides contained in
the gases are largely removed in the catalyst as the gases pass through
its catalytically acting fill. The denitrogenated gases then return
through duct 30 into the hot side of the layer of heat exchanging storage
masses whose surfaces are provided with catalytically active compounds,
and on through this layer. After being carried through the heat exchanging
storage mass 5a at the cold end, the exhaust gases have been cooled, by
the transfer of heat to the gas stream being carried into the
denitrogenation, to the temperature at which they are to enter the exhaust
chimney, and they are carried through the flue 32 to the exhaust chimney,
which is not shown.
EXAMPLE:
Gases from which nitrogen is to be removed issue at about
45.degree.-50.degree. C. from a wet desulfuration apparatus 1. In the next
heat exchanger 3, the gases moist from the wet desulfuration are heated to
about 70-90.degree. C. and fed through the connecting duct 22 to the
gas/gas heat exchanger 5 with a nitrogen oxide content of 1000 vpm. After
they are preheated in the heat storage mass 5a, when they strike the heat
exchanging storage mass 5b that is integrated with the hot side of the
gas/gas heat exchanger, and whose surfaces are provided with catalytically
active compounds, a first catalytic reduction of about 1% takes place. In
this reduction the reducing agent, preferably ammonia, that has been
absorbed from the largely denitrogenated gases by the side that is to be
cooled, is consumed. The gases enter the duct 24 connecting to the heater
7, with a temperature of about 300.degree.-320.degree. C. and with a
nitrogen oxide content of approximately 990 vpm. The gases are then heated
to 330.degree. to 35.degree. C. in the heater 7, by steam for example.
Approximately 950 vpm of ammonia is delivered through line 28 to the
exhaust gas ahead of the static catalyst 9, depending on the required
nitrogen oxide reduction. The exhaust gases leave the catalyst at
330.degree. to 350.degree. C. and about 60 vpm of nitrogen oxide,
corresponding to a reduction of about 94%, and still contain about 20 vpm
of unreacted reducing agent which is returned together with the gases
through duct 30 to the regenerative heat exchanger 5. The residual ammonia
is converted right at the entrance side by the heat-exchanging storage
masses 5b whose surfaces are provided with catalytically active compounds,
so that ammonia is absorbed and the nitrogen oxides are further reduced to
about 50 vpm, and transferred to the side corresponding to the gases from
which nitrogen is to be removed. After additional cooling within the heat
storage mass 5a on the cold side of the regenerative heat exchanger, the
denitrogenated exhaust gases pass free of reducing agents into duct 32 at
about 100.degree.-120.degree. C. and are carried into the chimney.
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